Hello, Dr. Z! So I saw an article in my feed that Hanford County (Maryland) is going to give out potassium iodide to anyone who lives closer than 10 miles from a nearby nuclear reactor. I don’t live near a commercial reactor, but there’s one at the state university just a few miles away. Should I ask the university or the city or state or county (or whoever) for some of this for me and my family?
So – the short answer is that you needn’t start a campaign or even a short letter asking that potassium iodide (KI) be distributed to people living near the university reactor. But I might need to explain some of the thinking that goes into saying so…beginning with why this is even a thought for people living near nuclear reactors. So here goes!
Let’s start with uranium fission – everyone’s favorite topic. When a uranium atom splits it forms two smaller, radioactive atoms, clustered around atomic weights of 100 and about 130; it turns out that iodine has an atomic weight of about 125 with I-129, I-131, and I-133 being produced in relatively high quantities in reactor fuel. So if there’s a reactor accident that causes the fuel to be damaged, there will be radioactive iodine released from the fuel – and into the atmosphere if the containment is damaged. Or, in the case of many small research reactors, if there is no containment. So the production (and possible release) of radioactive iodine is the first part of the answer.
The next part is that our thyroids soak up iodine from the bloodstream; most of the iodine that enters the body will end up in the thyroid within a day, irradiating it for weeks to months, depending on the amount of iodine absorbed by the body and the duration of the exposure. More iodine and/or a longer period of exposure means a higher thyroid dose. And since the thyroid is one of the most radiation-sensitive organs in the body, especially in children and young adults, this means that a serious nuclear reactor accident can expose a lot of thyroids to a lot of radiation exposure, possibly causing a lot of future thyroid problems, and maybe thyroid cancer as well. You may remember that, after the Chernobyl and Fukushima accidents, there was a great deal of attention given to checking children’s thyroids and even reports of some cancers being detected.
With all of this, there are two things to keep in mind: first, it takes several years for radiation-induced thyroid cancer to show up, so any tumors found in the first few to several years are almost certainly not from the reactor accident; second, there’s a background rate of thyroid nodules and cancers – finding someone with thyroid cancer doesn’t prove it was caused by the reactor accident; what we need to see is that, for example, there were 150 or 200 cases instead of an expected 100 cases of thyroid cancer in the decade following a reactor accident. Then we can say that the accident caused additional cancers, but we have no way to know which of the cases were caused by the accident and which would have occurred even if the accident had never happened.
OK – let’s get back to KI!
After a reactor accident, anyone exposed to radioactive iodine will run the risk of receiving a high radiation dose to their thyroid, but only if that iodine is absorbed. Which means that if we can flood the thyroid with non-radioactive iodine, all of the “landing sites” will be blocked by the stable iodine and the radioactive iodine will simply pass out of the body. But here’s the thing – some people have an allergy to potassium iodide, so we need to consider that taking KI carries with it a slight risk. In addition, adult thyroids are less sensitive to radioactive iodine than kids’. So we need to have some guidelines for its use. One of those guidelines is that KI is not recommended for anyone over the age of 40, nor is it recommended if the thyroid dose to be averted is less than 5 rem. Otherwise, the risk of an adverse allergic reaction is greater than the risk from radiation exposure.
So now we get to your question!
Compared to power reactors (not including the new SMRs), research reactors are tiny – a commercial reactor plant will generate more than 1000 million watts (1000 MW) of electricity, which requires producing three times that much thermal energy. The largest research reactor in the US, on the other hand, only produces about 10 MW of thermal energy – about 0.3% as much energy as the commercial reactors. Since each fission releases the same amount of energy no matter what kind of reactor it’s in this means that the country’s largest research reactor produces only about 0.3% as much radioactivity – and only about 0.3% of the iodine isotopes – as a commercial reactor such as the one in Maryland. This, in turn, means that the area that might be affected to the point of requiring KI to be distributed is tiny – depending on the exact location of the reactor on campus and the nature of the surrounding areas (public areas, university land, residential neighborhood, farmland) there might be only a handful of people who could meet the criteria of people for whom KI would be recommended. And if you’re over the age of 40, this is all a moot point as you wouldn’t be given KI anyhow because, due to the adult thyroid’s low sensitivity to radiation, the risk that you might be allergic to KI would outweigh the potential radiation dose. For these reasons, what might make sense for people living near two large nuclear reactors with a combined output of about 8000 MW of thermal energy and just under 2800 MW of electrical output (and the concomitant fission product inventory) might not make sense for those with a small research reactor as a neighbor.